Hydrate inhibition

ABSTRACT

A hydrate inhibitor comprising a blend of an additive (i) and at least one of an additive (ii) and an additive (iii) wherein additive (i) is a polymer of an ethylenically unsaturated N-heterocyclic carbonyl compound, with 6-8 ring atoms in the heterocyclic ring, additive (ii) is a corrosion inhibitor and additive (iii) is a salt of formula (I):[R 1 (R 2 )XR 3 ] + Y −  1/v, I wherein each of R 1 , R 2  and R 3  is bonded directly to X, each of R 1  and R 2 , which may be the same or different is an alkyl group of at least 4 carbons, X is S, NR 4  or PR 4 , wherein each of R 3  and R 4  which may be the same or different represents hydrogen or an organic group, with the proviso that at least one of R 3  and R 4  is an organic group of at least 4 carbons, especially at least 5 carbons, and Y is an anion of valency v, wherein v is an integer of 1-4.

The present invention relates to hydrate inhibitors and a method forinhibiting the formation of hydrates in particular to a method forinhibiting the formation of hydrates in the petroleum and natural gasindustries.

Hydrates are formed of two components, water and certain gas molecules,e.g. alkanes of 1-4 carbons, especially methane and ethane, such asthose found in natural gas. These ‘gas’ hydrates will form under certainconditions, i.e. when the water is in the presence of the gas and whenthe conditions of high pressure and low temperature reach respectivethreshold values. The gas may be in the free state or dissolved in aliquid state, for example, as a liquid hydrocarbon.

The formation of such hydrates can cause problems in the petroleum oiland natural gas industries.

Hydrate formation in the field may cause blocked pipelines, valves andother process equipment.

The problem is particularly of concern as natural gas and gas condensateresources are discovered where operating conditions surpass thesethreshold values, i.e. in deep cold water and on-shore in colderclimates.

Hydrates can also form in association with the underground hydrocarbonreservoir thus impeding production by blockage of reservoir pores.

The problem of hydrate formation is however commonest during gastransportation and processing, the solid hydrate precipitating frommoist gas mixtures. This is particularly true with natural gas whichwhen extracted from the well is normally saturated with water. Often insuch a case, in a cold climate, hydrates will form in downstreamtransportation networks and this can cause large pressure dropsthroughout the system and reduce or stop the flow of natural gas.

Hydrate formation may also occur during natural gas cryogenicliquefaction and separation.

A typical situation where hydrate formation can occur is in off shoreoperations where produced fluids are transported in a long verticalpipeline, for example, a riser system. Such produced fluids normallyinclude light gases known to form hydrates and water. In such asituation a temperature of 4.5° C. and a pressure of 150 psi would besufficient for hydrate formation.

Several methods are known to prevent hydrate formation and subsequentproblems in pipelines, valves and other processing equipment.

Physical methods have been used, e.g. increasing gas temperature in thepipeline, drying the gas before introduction into the pipeline, orlowering the gas pressure in the system. However, these techniques areeither expensive or are undesirable because of loss of efficiency andproduction.

Chemical procedures have also been used. Electrolytes, for example,ammonia, aqueous sodium chloride, brines and aqueous sugar solutions maybe added to the system.

Alternatively, the addition of methanol or other polar organicsubstances, for example, ethylene glycol or other glycols may be used.Methanol injection has been widely used to inhibit hydrate formation.However, it is only effective if a sufficiently high concentration ispresent since at low concentrations there is the problem of facilitationof hydrate formation. Also for methanol to be used economically undercold environmental conditions there must be early separation andexpulsion of free water from the well in order to minimise methanollosses in the water phase.

We have now found certain additives which may be used as effectivehydrate inhibitors at low concentrations.

The present invention provides a blend comprising Additive (i) which isa polymer of (a) an ethylenically unsaturated N-heterocyclic carbonylcompound, with 6-8 ring atoms in the heterocyclic ring and optionally(b) a different ethylenically unsaturated N-heterocyclic carbonylcompound with 5-7 ring atoms in the heteroring, the numbers ofheteroring atoms in (a) and (b) differing by at least one, and at least1 of Additives (ii) a corrosion inhibitor and (iii) a salt which is offormula [R¹(R²)XR³]⁺Y⁻1/v, 1 wherein each of R¹, R² and R³ is bondeddirectly to X, each of R¹ and R², which may be the same or different isan alkyl group of at least 4 carbons, X is S, NR⁴ or PR⁴, wherein eachof R³ and R⁴ which may be the same or different represents hydrogen oran organic group, with the proviso that at least one of R³ and R⁴ is anorganic group of at least 4 carbons, especially at least 5 carbons, andY is an anion of valency v, wherein v is an integer of 14 e.g. 1 or 2.

The present invention also provides a method of inhibiting or retardinghydrate formation and/or growth, which method comprises adding a blendof the invention in amount effective to inhibit or retard hydrateformation to a medium susceptible to hydrate formation.

The Additive (i) is a Polymer of (a) and optionally (b), each beingethylenically unsaturated N-heterocyclic carbonyl compounds. The weightproportions of structural units from (a) to (b) may be 100:0 or100-40:0-60, such as 100-60:0-40 or preferably 100-85:0-15, preferredproportions are 100:0, 50:50 and 75:25. The Polymer has a hydrocarbonchain with pendant N-heterocyclic carbonyl groups, with the bonding tothe chain preferably via the heteroring -N- atom.

The polymer may be made by polymerisation of (a) or simplecopolymerisation of (a) and (b) or may be a graft copolymer, e.g. fromgrafting (b) onto homopolymeric (a). Each N-heterocyclic carbonylcompound may contain 1 or more than 1 e.g. 2 or 3 heterocyclic rings,but each case it contains at least 1 ring containing the specifiednumber of ring atoms. That N heterocyclic ring may contain 1-3 ring Natoms but especially I ring N atom and 0-2 other ring hetero atoms e.g.0 or S, but especially no extra ring hetero atom. The ring or rings maybe saturated or ethylenically unsaturated. The carbonyl group may be inany position in the N heteroring, but is especially alpha to the Nhetero atom, so the N-heterocyclic rings are preferably derived fromlactams, such as those derived from butyric, pentenoic, pentanoic orhexanoic acid lactams (or 2-pyrrolidone, 2-pyridone, 2-piperidone oromega caprolactam). The polymer may have structural units from N-vinylomega caprolactam (and be a homopolymer) or may also have structuralunits from N-vinyl pyrrolidone.

The polymer preferably consists essentially of structural units derivedfrom (a) e.g. homopoly caprolactam or consists essentially of structuralunits from (a) and only ethylenically unsaturated N-heterocycliccompounds especially (b); structural units from polar ethylenicallyunsaturated non cyclic compounds (especially from esters of an alcoholcontaining more than I polar group and an unsaturated acid) and/orethylenically unsaturated carbocyclic carbonyl compounds are preferablysubstantially absent.

Preferably the Polymer is water soluble or water dispersible, e.g. to anextent of at least 0.01% by weight in water such as at least 0.05% butespecially at least 0.5%, such as up to 10% by weight. Its molecularweight is usually 5,000 to 1,000,000 e.g. 10,000 to 1,000,000 such as1,000 to 50,000 or 50,000-500,000 and preferably has a K value of 10-150especially 15-50, wherein the K value is obtained from the relativeviscosity in aqueous solution via the FIKENTSCHER'S Formula, from whichthe average molecular weight is calculated as described in U.S. Pat. No.2,811,499.

The polymers may be as described in WO 94/12761, the disclosure of whichis herein incorporated by reference.

The blend of the invention also comprises at least one of Additive (ii)the Corrosion Inhibitor and (iii) the salt of formula I. The blends maycomprise both (ii) and (iii), or may comprise (ii) in the substantialabsence of (ii) or (iii) in the absence of more than 2% of (ii) (basedon the combined weight of Additives (i) and (iii), preferably in thesubstantial absence of (ii).

The Additive (ii) is a corrosion inhibitor eg. for steel and usually onesuitable for use in anaerobic environments. It may be a film former,capable of being deposited as a film on a metal eg. a steel surface suchas a pipeline wall. It preferably has surfactant activity and especiallysurface wetting activity.

It is especially a nitrogenous compound with 1 or 2 nitrogen atoms. Thecorrosion inhibitor may be a primary, secondary or tertiary amine, or aquaternary ammonium salt, usually in all cases with at least onehydrophobic group, usually a benzene ring or a long chain alkyl groupeg. of 8-24 carbons. It may be a quaternary ammonium salt, a long chainaliphatic hydrocarbyl N-heterocyclic compound or a long chain amine. Thequaternary salt may be an (optionally alkyl substituted) benzyl trialkylammonium halide, in particular when at least 1 and especially 1 or 2alkyl groups is of 1-20, in particular 8-20 carbons such as cetyl andthe other alkyl groups are of 1-6 carbons such as methyl or ethyl;examples are benzyl alkyldimethyl ammonium chloride and Benzalkoniumchlorides e.g. mixtures of benzyl alkyl dimethyl ammonium chloridesespecially wherein each alkyl has 8-20 carbons, in particular 8-18 or12-18 carbons.

Other quaternary ammonium salts may be of formula [R⁵R⁶NR⁷R⁸]⁺Z⁻ 1/w,wherein Z is an anion eg. a halide or sulphate and w is an integer of1-4 e.g. 1 or 2, R⁵ is an alkyl or alkenyl group of at least 8 carbons,R⁶ is an alkyl or alkenyl group, each of at least 2 carbons or a N-heterocyclic group, and R⁷ and R⁸, which may be the same or differentrepresents an alkyl group, with the proviso that at least one of R⁶-R⁸has less than 4 carbon atoms. R⁵ may be of 8-24 carbons, such as 10-18carbons, especially, dodecyl, lauryl, cetyl, palmityl, stearyl or oleyl,while R⁶ may be selected from the same groups as R⁵, or may be ethyl,propyl, isopropyl, butyl or hexyl. R⁷ and R⁸ may be selected from thesame groups as R⁶ but preferably represent methyl groups. Examples ofthese quaternary salts are cetyl trimethyl ammonium, dodecyltrimethylammonium and lauryl trimethylammonium halides, eg. chlorides orbromides.

Other quaternary salt corrosion inhibitors are of formula [R⁹NR¹⁰R¹¹]³⁰Z⁻ 1/w where Z is a anion eg. as defined above and w is an integer of 14e.g. 1 or 2, R⁹N or R⁹NR¹⁰ forms a quaternizable N heterocyclic ring,and R¹¹ represents an alkyl or alkenyl group each of at least 8 carbonseg. as described for R⁵. The R⁹N group may be N- heterocyclic group with1 or 2 ring N atoms, especially with 1 or 2 heterocyclic rings, eg. of 5or particularly 6 ring atoms; examples of the rings are saturated oneseg. piperidine. The group R⁹NR¹⁰ may also be such an N heterocyclicgroup but with the R⁹ and R¹⁰ groups combined with the N atom to whichthey are bonded to form an unsaturated ring or fused N bridged ringsystem such as a pyridine ring. R¹⁰ if present may otherwise be an alkylor alkenyl group eg. as described for R⁸. Examples of these quaternariesare cetyl pyridinium halides, such as the chloride.

The corrosion inhibitor may also be a long chain aliphatic hydrocarbylN-heterocyclic compound, which is not quaternised. The aliphatichydrocarbyl group in the heterocyclic compound usually has 8-24 carbonsin the hydrocarbyl group, preferably a linear saturated or mono ordiethylenically unsaturated hydrocarbyl group; cetyl-, stearyl andespecially oleyl- groups are preferred. The N-heterocyclic compoundusually has 1-3 ring N atoms, especially 1 or 2 which usually has 5-7ring atoms in each of 1 or 2 rings; imidazole and imidazoline rings arepreferred. The heterocyclic compound may have the aliphatic hydrocarbylgroup on an N or preferably C atom in the ring; the ring may also havean amino-alkyl (e.g. 2-amino ethyl) or hydroxyalkyl (e.g.2-hydroxyethyl) substituent, especially on an N atom.N-2-aminoethyl-2-oleyl-imidazoline is preferred. The long chain amineusually contains 8-24 carbons and preferably is an aliphatic primaryamine, which is especially saturated or mono ethylenically unsaturated;an example is dodecylamine. Mixtures of any of the above corrosioninhibitors with each other may be used, eg a quaternary ammonium saltand a long chain aliphatic hydrocarbyl-N-heterocyclic compound (whereeach is preferably as described above), or mixtures with a tertiaryaliphatic amine.

If desired the corrosion inhibitor eg. a long chain amine may alsocomprise a phosphate ester salt, especially one with surface wettingactivity. Such phosphate esters are anionic surfactants, which are saltsof alkali metals eg. sodium or a quatemray ammonium eg. tetra methylammonium or tetrabutyl ammonium salts of acid phosphate esters, eg. with1 or 2 organic groups and 2 or 1 hydrogen atoms; examples of the organicgroups are alkyl or alkenyl groups as described for R⁵ above. Examplesof such phosphate ester salts are mono and dioctyl acid phosphate saltsand mixtures thereof. A preferred blend comprises a long chainalkylarine and a phosphate ester salt eg. as sold as NAL 1272 by Nalco.Other corrosion inhibitors include blends of a phosphate ester salt andan inorganic salt, usually with water and a glycol ether e.g. butyldiglycol ether such as is sold by BP Chemicals under Trade Mark C795.

Additive (iii) is a salt of formula I, [R¹R²XR³]⁺Y⁻ 1/v in which each ofR¹ and R² is an alkyl group of at least 4 carbons, which may be a linearalkyl or branched alkyl group, eg. a secondary or tertiary alkyl groupor especially an isoalkyl group. Each of R¹ and R² may be an alkyl groupof 4-24 carbons, preferably 4-10 and especially 4-6 carbons, such asn-butyl, isobutyl, secbutyl, tertiary butyl, n-pentyl, sec pentyl,isopentyl or tertiary pentyl group, or hexyl group. R³ and R⁴ (ifpresent as is preferred) are each hydrogen or an organic group eg. of1-24 carbons such as an alkyl or alkenyl group each preferably of 8-20carbons, eg. as is described for R⁵ above; however at least one or R³and R⁴ contains at least 4 carbons eg. at least 5 carbons, especially ina group with a linear chain containing such numbers of carbon atoms. R³may preferably be an alkyl group of 10-16 carbons, especially a mixtureof 2 or more such alkyl groups. R⁴ may be hydrogen but is preferablyalkyl of 1-10 carbons, eg. methyl or ethyl but especially of 4-6 carbonatoms, such as is described for R¹ or R², Y is an anion e.g. selectedfrom those described for X or Z above. Preferred salts are tetran-butyl, tetra n-pentyl, tetra-iso-pentyl ammonium (and phosphonium)halices eg. chlorides or especially bromides, and C₁₀₋₁₆ alkyl trin-butyl ammonium (and phosphonium) halides, especially chlorides orbromides. Tri n-butyl, n pentyl or isopentyl sulphonium halides eg.chlorides or bromides may be used.

The Additives (i) and (ii) may be used in weight ratios of 25:0.5-20e.g. 25:0.75-15 especially 25:1.5-15, 25:3-12 or 25:3-7, while theweight ratio of Additives (i) and (iii) may be 25:3-50 e.g. 25:15-40.When Additives (i), (ii) and (iii) are present, the relative weightratios may be 25:0.5-20:3-50, such as 25:1.5-15:15-40. The Additives maybe used in amounts to provide 1000-4000ppm Additive (i), 50-2000 e.g.150-2000 ppm and especially 300-700 Additive (ii) and 400-4000 ppmAdditive (iii) (based on the total weight of water present in themedium), preferably with total amounts of Additives ((i) and (ii)/(iii)as present) of 1500-8000 e.g. 4000-7000 ppm (on the same basis).

The Additives (ii) and (iii) may themselves be present in weight ratiosof 1-99:99:1 such as 10-90:90:10, but especially with a weight excess ofAdditive (iii) such as with a ratio of (ii) to (iii) of 10-45:90-55.

The Additives (ii) and (iii) for use as hydrate inhibitors arepreferably water soluble, e.g. to at least 10 g/l in water at 20° C.They may be used undiluted, but preferably are in solution such asaqueous solution, for example, as a solution in brine, or preferably analcohol, for example, a water miscible one such as methanol or ethanol.Preferably are used Additives (ii) and (iii), an aqueous solution ofwhich has a pH 1.5-12, e.g. 4-9, either naturally or after adjustment ofthe pH. Additives (ii) and/or (iii) may be used in alcoholic solution.

Each Additive is suitably injected at concentrations in the range 10 to20,000 ppm, e.g. 30 to 10,000 ppm, especially 50-1200 ppm based on thetotal water volume in the medium, in which hydrate formation is to beinhibited, in particular at concentrations in the range 200-1500 ppm forAdditive (ii) and 500-5000 ppm for Additive (iii). The amount ofmethanol, ethanol, or mono, di or tri ethylene glycol added relative tothe total water volume in the medium is usually less than 10%, e.g. lessthan 5% or 2%, but especially less than 10,000 ppm, eg 1000-8000 ppm.

The inhibitors may be injected at normal ambient conditions oftemperature and pressure.

There may also be present with the Additive (i) at least one Additive(ix) which is at least one water soluble polymer of a polarethylenically unsaturated compound and/or at least one Additive (x)which is a hydrophilic colloid, Additive (ix) being different fromAdditive (i). The Additive (ix) is usually water soluble to at least 10g/l at 20° C. and advantageously has a molecular weight of1000-1500,000, e.g. 5000-1,000,000, preferably 200,000-1,000,000 andespecially 400,000-900,000. The ethylenically unsaturated compound ispreferably a vinyl or methyl vinyl group, and the polar group may be analcohol, carboxylic acid, sulphonic acid or N-heterocyclic group,especially pyrrolidone. Preferred polar compounds are thus vinylsulphonic acid, acrylic and methacrylic acids and N-vinyl pyrrolidoneand “vinyl alcohol”. The polymers may be copolymers, but are preferablyhomopolymers of these polar compounds, especially polyvinyl alcohol(e.g. hydrolysed polyvinyl acetate), polyacrylates and polyvinylpyrrolidone (PYP). The amount of said polymer Additive (ix) is usually10-1000%, such as 50-300% or 90-250% based on the weight of the total ofAdditive(s) (i) and (ii).

The hydrophilic colloid (x) is an organic solid which is soluble inboiling water, e.g. to at least 10 g/l or dispersible in boiling waterand may be soluble (at least 10 g/l) or dispersible in water at 20° C.It usually absorbs water strongly, e.g. to at least three times such as3-15 times its weight of water at 20° C., and swells in water. It canform a colloidal solution or dispersion in water and may have an averagemolecular weight of at least 10,000, e.g. 100,000-10,000,000. It may bea polysaccharide, e.g. with at least 4 carbohydrate units, especiallyone with at least some galactose units, e.g. 20-60% of such units, andmay contain carboxylic acid residues, so that an aqueous solution ordispersion thereof can have an acidic reaction. The polysaccharide maybe a natural gum, e.g. guar, agar, arabic, locust bean, karaya, carob ortragacanth gum, or a cellulosic material, such as starch, which may beunmodified or modified as an alkyl ether, e.g. methyl or ethyl celluloseor hydroxyalkyl ether, e.g. hydroxyethyl cellulose or carboxy alkylatedstarch, e.g. carboxy methyl cellulose (CMC). The polysaccharide may alsobe a synthetic, e.g. biosynthetic gum, the result of a microbiologicalprocess, e.g. fermentation; xanthan gum, which can be made byfermentation of dextrose with Xanthomonas campestris cultures, which ispreferred, especially water soluble versions of xanthan gum. The colloidmay also be proteinaceous, in particular gelatin or carrageenan (aseaweed extract), e.g. x-carrageenan. The colloid may also be apolyuronic acid or salt thereof, e.g. sodium or ammonium salt or esterthereof, such as a hydroxy alkyl ester (e.g. of propylene glycol),especially with beta-D-mannuronic acid residues; alginic acid andespecially sodium alginate is preferred. The amount of Additive (x) maybe 10-1000%, eg 50-300% or 90-250% by weight based on the total weightof Additives (i) and (ii).

There may be present the Polymer as Additive (xii) with an aliphatic(N-heterocyclic carbonyl) polymer with a hydrocarbon backbone. It iswater soluble or water dispersible, eg to an extent of at least 0.01% byweight in water such as at least 0.05% but especially at least 0.5%,such as up to 10% by weight. Its molecular weight is usually 5000 to1000000 eg 10000 to 1000000 such as 1000 to 50000 and preferably has a Kvalue of 10-150 especially 15-50, wherein the K value is obtained fromthe relative viscosity in aqueous solution via the FIKENTSCHER'SFormula, from which the average molecular weight is calculated asdescribed in U.S. Pat. No. 2,811,499. The Polymer has a hydrocarbonchain with pendant N-heterocyclic carbonyl groups, with the bonding tothe chain via the heteroring -N- atom and the N-heterocyclic carbonylgroups as described further above. The aliphatic group or groups in thepolymer may be part of the hydrocarbon chain, or bonded to it or to theN-heterocyclic carbonyl ring; the aliphatic group may be linear orbranched and maybe alkyl eg of 1-40 eg 2-25 carbons or alkenyl eg of2-20 carbons, especially methyl, ethyl, butyl or octyl, tetradecyl,hexadecyl, octadecyl, elcosyl, tricosyl or ethylene, butylene oroctylene. The molar ratio of aliphatic group to heterocyclic carbonylgroup in the Polymer is usually 1:99 to 20:80 eg 5-15:95-85.

The Polymer may be a copolymer having repeat units derived from at leastone monomer which is an optionally alkyl substituted vinylN-heterocyclic carbonyl compound) and at least one monomer which is anolefin; this copolymer may be simple copolymer formed bycopolymerization of the monomers or a graft copolymer formed by graftingthe olefin onto a polymer of the N-heterocyclic monomer. The Polymer mayalso be an alkylated derivative of a polymer of an optionally alkylsubstituted (vinyl N-heterocyclic compound) especially a homopolymer ofsuch a compound.

The optionally alkyl substituted vinyl N-heterocyclic carbonyl compoundmay be of general formula:

R¹²R¹³C═CR¹⁴R¹⁵

wherein each of R¹³, R¹⁴ and R¹⁵, which may be the same or different,represents a hydrogen atom or an alkyl group eg of 1-20 carbons, such asmethyl, ethyl, butyl, hexyl, decyl or hexadecyl, and R¹² represents anN-heterocyclic carbonyl group with the free valency on the N atom,preferably the N heterocyclic carbonyl group is as described above. TheN-heterocyclic ring may contain 1-3 ring N atoms but especially 1 ring Natom and 0-2 other ring hetero atoms eg 0 or 5, but especially no ringhetero atom; the ring may contain in total 1 or 2 rings, which may besaturated or ethylenically unsaturated such as a pyrrolidine,piperidine, quinoline or pyridine ring. Preferably R¹³, R¹⁴ and R¹⁵ arehydrogen and R¹² represents an N-(pyrrolidone), N-(2 pyrid-2-one) orN-(piperid-2-one) group.

The olefin is usually of 2-32 eg 4-18 carbon atoms and is generally ahydrocarbon. It is preferably an alkene, especially a linear alkene andhas in particular a terminal olefin group. It is preferably a vinylolefin eg of formula CH₂═CH—R¹⁶, where R¹⁶ is hydrogen or alkyl of 1-40carbons, such as methyl, ethyl, propyl, butyl, hexyl or decyl, tetradecyl, octadecyl or octacosyl (so the olefin is tricosene). The olefinis preferably butylene octene-1 or dodecene-1, hexa decene- 1,octadecene-1, eicosene-1 or tricosene-1.

The Polymer may be made by free radical copolymerizing theN-heterocyclic carbonyl compound eg N-vinyl pyrrolidone with the olefineg butylene in solution in the presence of a peroxide catalyst. ThePolymer may also be made by free radical grafting of the olefin onto apolymer of the N-heterocyclic carbonyl compound eg poly vinylpyrrolidone (PVP) with K value as described above. The copolymerizationsmay incorporate structural units from the olefin into the hydrocarbonpolymer chain and/or insert such units into the N-heterocyclic rings.

The Polymer may also be made by direct alkylation of the polymer of theN-heterocyclic carbonyl compound eg PVP with an alkylating agent eg analkyl halide such as butyl bromide or octyl bromide, optionally in thepresence of a base, such as triethyl amine.

Finally the Polymer may be a homo or copolymer of an alkyl substitutedN-(alkenyl) heterocyclic compound in which the alkyl substituent may bein the N-heteroring and/or present in the alkenyl side chain; the alkylsubstituent may be as is preferred for the aliphatic group on thePolymer described above. The Polymer may have structural units from anN-vinyl-alkyl ring substituted heterocyclic carbonyl compound, such asN-vinyl-3-methyl pyrrolid-2-one and/or from an N-butenyl-heterocyclecarbonyl compound, such as N-butenyl-pyrrolid-2-one. Such Polymers maybe made by polymerization in solution in the presence of a free radicalcatalyst, in an analoguous way to polyvinyl pyrrolidine.

Thus the preferred Polymers are aliphatic (N-heterocyclic carbonyl)polymers with units derived from N-vinyl pyrrolid-2-one and butylene(sold as Antaron P 904), octylene, dodecylene, hexadecylene (sold asAntaron V216), eicosylene and tricosylene; the Antaron products are soldby International Speciality Products of Wayne, N.J., USA. Polymer xiimay be as described in WO 93/25798 the disclosure of which is hereinincorporated by reference.

The Polymer may also be as Additive (xiii) a copolymer of (a) at least 1ethylenically unsaturated N- heterocyclic ring compounds especially withat least one of (b) a different ethylenically unsaturated N-heterocyclicring compound, (c) an ethylenically unsaturated carbocyclic carbonylcompound, and (d) a polar ethylenically unsaturated compound, differentfrom said N-heterocyclic compounds, said copolymer being different fromAdditive (i). Polymer Additive xiii preferably consists essentially ofstructural units derived from at least 2 of said (a)-(d) compounds, inparticular (a), (b) and (d). Polymers (xiii) from heteroring ringcompounds with different size rings e.g. with differences of 1-3 in thenumbers of ring atoms are preferred, especially ones with 5 and 7membered heterorings. Examples of the ethylenically unsaturatedN-heterocyclic ring compounds are ones described above in relation toPolymers (i) and (ix) especially N-vinyl-pyrrolidone and N-vinyl omegacaprolactam. The carbocyclic compound may be one with 4-8 eg. 6 or 7ring atoms with the carbonyl group in the ring preferably adjacent to acarbon atom carrying the ethylenically unsaturated group which ispreferably an alkenyl group of 2-6 carbon atoms especially with aterminal CH₂═CH-group. The carbocyclic compounds are cyclic ketones withan unsaturated side chain. Examples of the carbocyclic compound are2-vinyl- cyclohexanone and 2-vinyl-cycloheptanone. The polarethylenically unsaturated compound may be an ester of an alcoholcontaining more than 1 polar group and an unsaturated acid. The alcoholusually contains a hydroxyl group and at least one other hydroxyl groupor aminogroup, which may be a primary secondary or especially tertiaryamino group in particular in a non cyclic arrangement, thus the alcoholmay be a diol or an amino alcohol, especially an aliphatic one, such asa dialkylaminoalkanol, with 1-4 carbons in each alkyl and 2-4 carbons inthe alkanol. 2-Dimethylamino ethanol is preferred. The unsaturated acidis usually an aliphatic alkenoic acid with 3-1 0 carbons such as acrylicor methacrylic or crotonic acid. This unsaturated ester is especially a(meth) acrylate ester of a dialkylamino alkanol in particulardimethylaminoethyl acrylate or methacrylate. Polymer Additive xiii maybe as described in WO94/12761, the disclosure of which is hereinincorporated by reference.

The Polymer xiii may contain structural units in molar % derived from10-90% of the N-heterocyclic compound, 10-90% of the carbocycliccompound and 10-90% of the polar ethylenically unsaturated compound. Apreferred compound is a copolymer of N-vinyl pyrolidone, N-vinyl-omegacaprolactam and dimethylamino ethyl methacrylate, such as is sold asAntaron VC713 by International Speciality Products of Wayne, N.J.

The amount of said Polymer (xiii) is usually 10-1000% such as 50-300% or90-250% based on the weight of the total of Additives (i) and (ii).Mixtures of the aliphatic (N-heterocyclic carbonyl) polymer (xii) andthe copolymer (xiii) may be used especially in weight ratios of10-90:90-10 in particular a majority of(xiii) eg. in weight ratio of(xii) to (xiii) of 10-40:90-60.

Preferably at least one and preferably all of Additives ix, xii andxiii, and possible x are substantially absent from the blends andmethods of the invention.

The Formulations preferably contain at least one anti-foaming agent(xiv) especially when the Additive (ii) or (iii) has foaming activity,eg. when either contains at least one alkyl group of at least 8 carbonatoms. Examples of suitable anti-foaming agents are silicon containingcompounds especially organosilicon oxygen or nitrogen compounds, such aspolysiloxanes, including cyclic polysiloxanes, silicon polyethers,polysilazanes and fluorosiloxanes. Alkylpolysiloxanes are preferredespecially dimethyl polysiloxanes such as 3556 from Th. Goldschmidt KGand AF 1520 from Dow Corning. The silicon anti-foaming agents areusually used as water dispersible emulsions. Silicon free anti-foamingagents, such as suds depressants used in detergents, may also be used.The amount of the anti-foaming agent is usually 10-70 ppm e.g. about 40ppm (based on the total water present), or 0.1-5% e.g. 0.5-3% (based onthe total weight of Additives (ii) and (iii)).

Formulations comprising Additive (i), (ii) and/or (iii) and optionallyat least one of Additive (ix), (x), (xii) and (xiii) may be used intotal amount of 50-10,000 ppm. especially 150-2000 ppm, or2000-8000 suchas 4000-6000 ppm relative to the total water in the medium in whichhydrates may form (including any water added in the formulation).

The Formulation may also contain another hydrate inhibitor and/or awater dispersant or surfactant, in particular an anionic one such assodium dodecyl sulphonate or stearic acid and in amount of 1-10% of theFormulation weight and/or a biocide, e.g. formaldehyde, e.g. in amountof 10-10,000 ppm and/or a metal complexant such as citric acid (e.g. inamount of 10-10,000 ppm) all amounts being in relation to the totalweight of the Formulation.

The Formulations may be used to retard or inhibit hydrate formation andmay also reduce the rate of crystal growth of gas hydrates.

The inhibitor Formulations of the present invention are suitable for usein media containing water and gas, in particular in the petroleum,natural gas and gas industries. The gas may be a hydrocarbon normallygaseous at 25° C. and 100 KPa pressure, such as an alkane of 1-4 carbonatoms eg methane, ethane, propane n or isobutane, or an alkane of 2-4carbon atoms eg ethylene, propylene, n- or isobutene; the gas preferablycomprises by weight (or especially by moles) at least 80% and especiallyat least 90% of methane with 0.1-10% eg 1-5% C₂ hydrocarbon and/or0.01-10% eg 0.05-5% C₃ hydrocarbon. A natural gas, which may or may nothave been purified or processed is preferred. The gas may also containnitrogen eg in amount of 0.01-3% by weight and/or carbon dioxide eg inamount of 0.1-5% such as 0.5-2% by weight. The formulations of theinvention are particularly suitable for treating wet gases, whosecomposition (on a dry basis) comprises (by moles) 80-90% methane, 3-8%ethane, 1-5% propane and 0.5-3% C₄ and C₅ hydrocarbons, as well as 1-5%carbon dioxide and 0.1-1.5% nitrogen, these gases being particularlyprone to producing gas hydrates. The Additive (i) and (ii)/(iii) can bemore effective than poly vinyl pyrrolidone in the inhibition of gashydrate formation, especially when the gas comprises carbon dioxide.

In particular, they may be suitable for use during the transportation offluids comprising gas and water eg from oil or gas wells. They may alsobe suitable for use in oil based drilling muds to inhibit hydrateformation during drilling operations.

In another aspect therefore the invention provides an oil based drillingmud, which comprises as hydrate inhibitor at least one blend orFormulation of the invention.

When used during the transportation of fluids, e.g. gases with water andoptionally oil eg. condensate in conduits such as pipelines theinhibitors may be injected continuously or batchwise into the conduitupstream of conditions wherein hydrate formation may occur. Conditionsunder which gas hydrates may form are usually at greater than −5° C. eggreater than 0° C. such as 0 to 1 5° C. eg 1-10° C. and pressures eg of0.1-30 MPa eg 1-15 MPa the temperature of onset of gas hydrate formationdepending on the pressure, and the presence of, and concentration of,salt in the water. As the temperature decreases and the pressureincreases and the concentration of salt decreases the greater is thelikelihood for hydrate formation to happen in the absence of theAdditives and Formulations of the invention. Thus the conditions of useof the Formulations are ones such that in their absence a gas hydratemay form, or crystals of gas hydrate may grow In the absence of theFormulations ice may also form in addition to the gas hydrate especiallyat temperatures of −5° C. to 5° C. The pH of the water eg in thepipeline after addition of the Formulation is usually 3-9, especially3.3-5 or 5-7.5.

In drilling operations the inhibitors may be added to the drilling mudsin the mud tank at the wellhead.

The invention is illustrated in the following Examples.

EXAMPLES

To assess the efficiency of hydrate inhibitors suitable for use in themethod of the present invention, tests were carried out using thefollowing procedure:

The hydrate inhibitor test apparatus consisted of a simple 316 stainlesssteel pressure cell, with a usable internal volume of 1000 cm³ with athermostated cooling jacket, a sapphire window, an inlet and outlet anda platinum resistance thermometer. The cell contained water which wasstirred by a magnetic pellet. Temperature and pressure were monitoredand the results provided on a computer data logger; gas hydrates werealso detected visibly using a time lapse video recording system. Beforeeach test the cell was cleaned thoroughly by soaking successively in 10%aqueous hydrochloric acid for 1 hour, 10% aqueous sodium hydroxidesolution for 1 hour and then double distilled water.

Into the cell was placed 200 cm³ of pre-chilled double distilled waterwith or without the chemical to be tested. A PTFE stirrer pellet wasthen placed in the cell and the pH of the solution measured withsubsequent adjustment if desired by the addition of small butconcentrated amounts of hydrochloric acid or sodium hydroxide. Aftersealing the cell the water was then stirred at 500 rpm and allowed tocool to the operational temperature of 4° C. When this temperature wasreached the stirrer was stopped and the video recorder started. A gasmixture of 2% propane and 98% methane (by moles) was then admitted tothe cell until the pressure reached 70 bar (7 MPa) and the temperature,pressure and time were noted. The stirrer was restarted to run at 500rpm and the time noted. Hydrates were observed to form in the vesselwhen the solution in the vessel turned opaque, coincident with which wasa sharp temperature increase of about 0.2° C. and a gradual pressurereduction. The time from first contact of water and gas to formation ofhydrate was read from the logger.

The experimental conditions are very severe and accelerated test of gashydrate formation and inhibition. The amounts of the Additive areexpressed in ppm based on the volume of water. The inhibition timeresults given are an average of several results.

In the Examples, the following Additives were used

Additive (i)(a) was homopolymeric N-vinylomega caprolactam,

Additive (i)(b) was a copolymer with structural units from N-vinylcaprolactam and N-vinyl pyrrolidone in 75:25 molar ratio,

Additive (i)(c) was a copolymer with structural units from N-vinyl omegacaprolactam and N-vinyl pyrrolidone in a 50:50 molar ratio.

Additives (i)(a)-(c) were water soluble polymers obtained as about 52%solutions in methanol from International Speciality Products of WayneN.J., USA under the Trade Marks ACP 1177, 1160 and 1161 respectively,and with weight average molecular weights of about 75000, 128000 and206000.

Additive (ii)(a) was Benzalkonium chloride, which was a mixture ofbenzylalkyldimethyl ammonium chlorides from Fluka believed to have 8-18Calkyls.

Additive (ii)(b) was a corrosion inhibitor, sold by BP Chemicals underthe Trade Mark C 795, which was a mixture of phosphate ester salt, andan inorganic salt in water with butyl diglycol ether.

Additive (ii)(c) was a corrosion inhibitor (sold under the Trade Mark“Champion” RU189 by Champion Technologies Inc., Tex., USA), which isbelieved to be a 1:1 mixture of quaternary ammonium salt and analiphatic imidazoline.

Additive (iii) was tetra-n-pentylammonium bromide.

Additive (xiv) was an anti-foaming agent, which was apolydimethylsiloxane sold by Dow Corning under the Trade Mark AF1520.

Examples 1-8

The formulations were made up and added to the water so it contained theAdditives below in amounts (in ppm relating to total water present). Theresults (averaged over several repeat experiments) were as follows

Additive, Amount (ppm) Inhibition EXP. (i) (ii) (iii) (xiv) Total pHTime/Mins 0 — — — — — — 6 1 (a)2500 (a)1000 2500 — 6000 5.6 1650 2(a)2500 (a)500  2500 40 5540 4.8 1990 3 (a)2500 (b)500  2500 40 5540 6.5198 4 (a)2500 (c)400  2500 40 6040 6.4 367 5 (a)2500 (c)200  2500 405240 — 194 6 (a)2500 2500 5000 5.5 1120 7 (b)2500 2500 5000 — 508 8(c)2500 2500 5000 5.1 152 The weights of Additive (i) are expressed asweights of polymer, not weights of polymer solution.

EXAMPLE 9

The process of Ex. 1 was repeated at 5° C. with a North Sea gas mixturecomprising (on a dry basis by mol %) nitrogen 0.81%, carbon dioxide3.36% methane 84.4%, ethane 6.79%, propane 2.54%, isobutane 0.43%, nbutane 0.77%. iso pentane 0.245%, and n pentane 0.246% and the remainderhigher hydrocarbons. In addition there was present in the cell crude oilfrom a gas field from where the gas mixture came; in the cell there wereequal weights of water and oil. The process was also repeated with noAdditive. The time to hydrate formation was 0.5 min. for the latter andan average of 20 hr 41 mins. (1241 min.) when the Additives werepresent.

What is claimed is:
 1. A blend comprising an additive (i) and anadditive (iii) a salt of formula [R¹(R²)XR³]⁺Y⁻ 1/v,l wherein each ofR¹, R² and R³ is bonded directly to X, each of R¹ and R², which may bethe same or different is an alkyl group of at least 4 carbons, X is S,NR⁴ or PR⁴, wherein each of R³ and R⁴ which may be the same or differentrepresents hydrogen or an organic group, with the proviso that at leastone of R³ and R⁴ is an organic group of at least 4 carbons, and Y is ananion of valency v, wherein v is an integer of 1-4, wherein additive (i)is a polymer consisting essentially of structural units derived from anethylenically unsaturated N-heterocyclic carbonyl compound with 6-8 ringatoms in the heterocyclic ring.
 2. A blend according to claim 1 in whichadditive (i) is a homopolymeric N-vinyl-omega caprolactam.
 3. A blendaccording to claim 1 further comprising an additive (ii) a corrosioninhibitor.
 4. A blend according to claim 3 in which additive (ii) is atleast one of a primary, secondary or tertiary amine, a quaternaryammonium salt which differs from additive (iii) a long chain aliphatichydrocarbyl N-heterocyclic compound, a long chain amine or a blend of aphosphate ester salt and an inorganic salt.
 5. A blend according toclaim 1 in which additive (iii) is a tetra-n-butyl, tetra-n-pentyl,tetra-iso-pentyl ammonium or phosphonium halide.
 6. A blend according toclaim 5 in which additive (iii) is tetra-n-pentyl ammonium bromide.
 7. Ablend according to claim 1 which also comprises at least one of additive(ix) which is a water soluble polymer of a polar ethylenicallyunsaturated compound, additive (x) which is a hydrophilic colloid,additive (xii) which is an aliphatic (N-heterocyclic carbonyl) polymerwith a hydrocarbon backbone, additive (xiii) which is a copolymer of (a)at least one ethylenically unsaturated N-heterocyclic ring compound withat least one of (b) a different ethylenically unsaturated N-heterocyclicring compound and (c) a polar ethylenically aliphatic unsaturatedcompound, different from said N-heterocyclic compounds, said polymer(xiii) being different from additive (i), and additive (xiv) which is ananti-foaming agent.
 8. A blend according to claim 1 in which additive(i) is water soluble.
 9. A blend according to claim 3 in which therelative weight ratio of additive (i): (ii): (iii) is 25:0.5-20:3-50.10. A method of retarding or inhibiting hydrate formation which methodcomprises adding a blend as claimed in claim 1 to a medium susceptibleto hydrate formation.
 11. A method according to claim 10 wherein thetotal additive concentration is 50-10,000 ppm relative to the totalwater in the medium.
 12. A method according to claim 10 wherein themedium is in the presence of water and a gas which comprises methane andat least one of 0.1-10% C2 hydrocarbon and 0.1-10% C3 hydrocarbon and0.1-5% carbon dioxide, all by weight.
 13. An oil-based drilling mudcomprising a blend as claimed in claim
 1. 14. A blend according to claim4 in which additive (i) is a homopolymeric N-vinylomega caprolactam andadditive (iii) is tetra-n-pentyl ammonium bromide or tetra-n-butylammonium bromide.
 15. A method of retarding or inhibiting hydrateformation which method comprises adding a blend as claimed in claim 3 toa medium susceptible to hydrate formation.
 16. An oil-based drilling mudcomprising a blend as claimed in claim 3.